Why? Can't you implement your own website into XenForo? It would make things 1000x easier
I guess the easiest way to do it would be to make XenForo take cookies from the same domain as the main site (I'm already assuming it's on the same domain, so that's no too easy).
You could either link to the XF login form, or simply request the login details, check them yourself with the XF classes and set the cookie, or get the login details and post them to the XenForo login, then set the cookies you get back to the users browser.
You could then include the XenForo system in your PHP, and use it to get the relevant details.
I can tell you that we went through this on our PHP/CodeIgniter application and it took over 30 hours of development to get external logins working with XenForo, and still, accessing the AdminCP requires a kludgy workarond.
XenForo needs to provide a way for a third party script to say "This user is logged in". Not, here's the user's password, let's juggle hashing algorithms and go ten rounds. Just, this user (e-mail address) is logged in so get over it!
SMF, a free forum, provides exactly this functionality through a third party script. We got external logins completely working in less than 2 hours. However we wanted to use a professional forum and so endured rewriting all this in XenForo.